Radio frequency (RF) transceivers are found in many two-way communication devices, such as portable communication devices, (cellular telephones), personal digital assistants (PDAs) and other communication devices. A RF transceiver must transmit and receive RF signals dictated by the particular communication protocol under which the communication device is operating. For example, RF communication protocols include amplitude modulation, frequency modulation, phase modulation, or a combination of these. A typical code-division multiple-access (CDMA) communication system, uses a direct-conversion receiver with a matching filter to separate the appropriate data or output signal from the received baseband signal.
A direct-conversion receiver, also known as a homodyne, synchrodyne, or zero-intermediate frequency (IF) receiver, is a radio receiver design that uses a unique system of demodulating amplitude-modulated (AM) signals. It uses a local oscillator, synchronized in frequency to the carrier of the desired signal, to modulate the received signal. Thereafter, the matching filter is used to separate the appropriate output signal from the received baseband signal.
Consequently, filter tuning or adjusting is often performed in direct-conversion and low-IF receivers. It is well established that it is more convenient to design continuous-time filters for channel selection instead of switched-capacitor filters due to physical area requirements and dynamic range constraints of the respective filter circuits. However, continuous-time filters require tuning since their cut-off frequency depends on a factor (i.e., a resistor-capacitor (RC) time constant), which is sensitive to manufacturing process variation and operating temperature of the various components used to implement the filter.
In CDMA applications, an in-band blocker profile, for a receive channel, dictates that the cut-off frequency should be accurate to within ±2.5% of the nominal cut-off frequency. A first simulated variation of cut-off frequency for a continuous-time filter due to process variation shows that the cut-off frequency of an active continuous-time filter will vary by more than ±10.0% of the nominal cut-off frequency due to process variation. A second simulated variation in cut-off frequency for a continuous-time filter due to temperature variation is less than ±1.0%. Consequently, a tuning scheme that neutralizes the effects of at least process variation on the cut-off frequency of a continuous-time filter is desired.
There are several conventional ways to tune continuous-time filters. One common way is to construct a master voltage-controlled oscillator (VCO) whose oscillation frequency is proportional to the slave's (i.e., the matching filter's) cut-off frequency. The tuning, in this case, is performed by a phase-locked loop (PLL). Typically, the filter comprises a combination of an operational amplifier with resistors and capacitors arranged in a feedback path. In this arrangement, a digital PLL can be used as one or both of resistance and capacitance can be adjusted by discretely switching select resistors and capacitors in the feedback path. This method provides good accuracy at the expense of the additional circuit area required implementing the VCO and the relatively large components required to bring the matching of the master and slave filters to a desired level.
A second way to tune a continuous-time filter is by estimating an RC-time constant on an integrated circuit. This is generally accomplished by charging an RC load with a voltage or current source and adjusting the RC-time constant to achieve a desired response. This method is attractive due to its simplicity. However, for high-frequency applications, this method suffers from sensitivity to component mismatch and amplifier offsets.
The CDMA standard presents at least two additional problems. First, the CDMA standard does not provide a time slot to check and/or adjust filters. Second, the CDMA standard dictates that filter performance must not be degraded while the host communication device is operational.